1. Field of the Invention
The present invention relates to a radio paging system and, particular, to a global radio paging system in which a plurality of radio frequency carriers having same frequency and modulated synchronously with same digital selective call signal, that is, paging signal, are transmitted from a plurality of mutually remote base stations, respectively.
2. Description of the Prior Art
In a radio paging system, in order to make a service area thereof wider, it is usual to arrange a plurality of base stations which are mutually separated by a predetermined distance and each of which has a radio transmitter whose output power is in the order of 100 W, and to simultaneously transmit a plurality of radio frequency carriers having same frequency and modulated with same digital paging signal therefrom. In such radio paging system, it is necessary to limit a phase difference between two received carriers in an area in which such radio frequency carriers from adjacent two base stations can be received, that is, an interference area, to a value not higher than a constant value which is usually within one fourth of a bit interval of the digital paging signal. For example, in a paging system which utilizes a digital paging signal of 512 BPS standardized by POCSAG (Post Office Code Standardization Advisory Group), that is, POCSAG code, the above mentioned phase difference must be not more than 488 .mu.s. Therefore, it is necessary to make a phase error from a reference phase in each base station not more than about 250 .mu.s.
On the other hand, a paging system of this kind is equipped with a central station between the base stations and a telephone switching network receiving paging requests. The central station includes a paging controller for performing a format conversion of paging request data from subscribers into a predetermined signal format, for example, a paging request signal format based on POCSAG code.
Interfaces between the central station and the base stations are formed by cables or radio frequency time-division multiplexors. In order to restrict the phase error within the predetermined range (about 250 .mu.s) by compensating for a delay caused by signal transmission through the connection circuit between the central station and the base stations, the central station and the base stations include variable delay means respectively. On the other hand, U.S. Pat. No. 4,709,401 discloses an example of a paging system in which a central station transmits a radio frequency carrier signal modulated with a phase correction code having a predetermined code pattern to base stations and one of the base stations which receives the carrier signal and demodulates it to the phase correction code calculates and sets an amount of delay of its own variable delay means on the basis of the phase correction code.
In general, since the above mentioned connection circuit between the central station and one of the base stations is composed of a usual route and an extra route, it is necessary in the phase synchronous system of the U.S. patent to retry the calculation and setting of the amount of delay when the usual circuit is switched to the extra circuit due to failure in the usual circuit.
Further, when bit rate of the digital paging signal is further increased in order to satisfy recent request for effective utilization of radio frequency, the accuracy of phase correction according to the conventional technique mentioned above is not enough. That is, accuracy of phase correction required in a radio paging system corresponding to POCSAG code bit rate of which is 9600 BPS should be not more than 26 .mu.s. This means that phase correction accuracy at a modulation input of a transmitter at which difference in delay between radio transmission areas and/or phase error in the transmitter is produced unavoidably must be within about 10 us which is practically impossible. Further, in the prior art mentioned above, a variation of delay in a transmission line connecting the central station to the base station must be made smaller than the above mentioned tolerable phase error. However, it is very difficult to achieve such accuracy in the time-division multiplexor circuit. An increase of signal delay in separating a time-division multiplexed signal in the above mentioned transmission line is always varied due to the fact that clock pulses of the paging controller and the time-division multiplexor are usually independent from each other. Therefore, if, in order to restrict a variation of delay, time-division multiplexing is restricted, the number of radio transmission lines is necessarily increased, causing cost for provision and maintenance of the transmission lines to be increased.
Further, an increase of the number of base stations in order to expand a service area requires an increase of the number of connection circuits between a central station and base stations, which increases cost for provision and maintenance of such circuits.
A multi-drop system in which a plurality of base stations are connected to a single line and a tree-connection system in which a signal distributor is arranged between a central station and base stations are effective in solving this problem. However, in the multi-drop system, a construction of transmission line is changed at a point on the line between the central station and the base station and, therefore, when an amount of delay is changed, a phase correction becomes very difficult. That is, although a variation of de ay occurred in a portion of the transmission line on the side of the central station does not arise any problem since phases of all of the base stations connected thereto change similarly, a variation of delay occurred on the side of base stations causes phase error between them. In the tree system, although it is possible to reduce a total length of transmission lines, a variation of de ay is increased correspondingly to the provision of the signal distributor. Therefore, when bit rate of a digital paging signal is increased, it is difficult to mantain phase synchronization between transmission outputs of a plurality of base stations. When a plurality of signal distributors are used, phase correction in base stations arranged in downstream side of the distributors becomes difficult.